Yol. 67.] SKOMER VOLCANIC SERIES. 207 



These clots are circular or elliptical in outline, are separated from 

 each other, and measure up to 3 mm. across. 



The olivine exists as minute serpentinous pseudomorphs exhibit- 

 ing borders of iron-oxides. Iron-ores with regular sections are 

 plentiful, and apatite is an abundant accessory. 



The coarse dolerite (fig. 12, B, p. 206) consists of a pale purple 

 augite in ophitic plates, with laths and lath-shaped porphyritic 

 crystals of labradorite-felspar. It contains fairly large pseudo- 

 morphs after olivine in serpentine, chlorite, and delessite, or the 

 green micaceous mineral mentioned above (p. 199), veins of which 

 also traverse the rock. 



Chemically, the Skomer dolerites are closely related to the 

 Jedburgh type of dolerite of Central Scotland, described by Prof. 

 W. W. Watts x and later by Mr. E. B. Bailey, 2 and would have a 

 silica -percentage of about 46. The titanium, iron, lime, and 

 magnesia would all be high, but the percentage of phosphoric acid 

 is probably greater than in the Jedburgh dolerites. 



Structurally, the Skomer rocks are almost identical with a rock 

 from a point 700 yards west of Dumbeath, Stirling Castle (S 12568), 

 but are slightly less rich in olivine. 



VI. The Associated Clastic Rocks. 



The sediments associated with, and occurring within, the Volcanic 

 Series form one main group, about 450 feet thick, which stretches 

 in a strong ridge across Skomer (see map, PI. XI). These rocks 

 are thrown northwards by a fault which passes from South Haven 

 to North Haven and between the Rye Rocks. On the mainland 

 these sediments form a much-faulted belt near the northern coast 

 between Martin's Haven and Hopgang, and also occupy Cable Bay 

 and Mouse's Haven in the Wooltack peninsula. They are well 

 exposed on Skomer in the Wick, at Welsh AY ay and at the base of 

 High cliff, in North and South Havens, and at Protheroe's Dock on 

 the north side of the Neck. They consist, in the lower part, of 

 coarse rhyolitic breccias and conglomerates alternating with white, 

 buff-coloured, or green quartzites ; in their upper part, they consist 

 of alternating quartzites and red clays — the latter becoming pre- 

 dominant at the top. These red clays are exceptionally well 

 displaved in the sections at the Wick, South Haven, and Protheroe's 

 Dock.' ; 



Quartzites belonging to other horizons in the series occur in 

 the extreme south of Skomer and in the Mewstone Channel ; below 

 the dolerite of Pigstone Bay and the trachyte of North Castle ; at 

 South Castle on the Neck ; on the south side of Midland Island ; 

 on the north side of the Deer Park at Wooltack; on the high ground 

 west of Martin's Haven ; and as a thin band on the northern coast 



1 In Sir Archibald Geikie's ' Ancient Volcanoes of Great Britain ' vol. i (1897) 

 p. 418. 



2 ' Geology of the Country around Glasgow ' Mem. Geol. Surv. [in the press]. 



3 For a detailed description of these sections, see E. W. Small. 1899-1909. 

 pp. 12-13. 



Q. J. G. S. No. 266. p 



